A standard hammer drill has a drive spindle that is rotated about a spindle axis and a hammer displaceable axially in the drive spindle. A chuck is carried on the spindle so as to rotate therewith, and holds a drill bit having a shaft or shank that may be cylindrical, of hexagonal section, or formed with a radially outwardly open and at least generally axially extending groove. The chuck holds the drill bit so it is centered on the spindle axis, and the hammer is effective on the rear end of the drill bit. The bit is rotationally coupled to the chuck body, which as mentioned above is rotated, and is at least limitedly axially reciprocated by the hammer so that it can drill through masonry or the like. The jaws of the chuck are normally urged radially outward by springs and are displaced radially inward by an adjustment ring which has threads that mate with teeth on the jaws.
When such an arrangement is used with an ungrooved hexagonal-section drill bit there are as described in German utility model No. 7,227,714 three angularly equispaced jaws and each jaw has an inner end formed with a pair of radially inwardly pointing teeth. Both teeth of each tooth engage a respective one of the facets of the drill-bit shank to hold it on center and in place in the chuck against rotation relative thereto. When tightened on the shank of the bit, these jaws rotationally solidly couple the bit to the chuck body, with relatively solid six-point contact and with some axial sliding relative to the jaws still being possible.
Such an arrangement wears rapidly, the effective edges of the teeth quickly dulling and becoming no longer able to hold solidly on the drill-bit shank. In addition such a bit can normally only be used with ungrooved cylindrical-shape bit shanks that are fairly small, and this is done by providing between the two teeth forming the engagement flanks a third tooth set radially somewhat outward, that is back from the two teeth flanking it. In such an arrangement the surfaces extending from each tooth edge serve no useful function.
German patent document No. 2,842,783, (British equivalent patent No. 2,030,485) and German utility model No. 8,337,365 describe an arrangement particularly useful in hammer drills. Here the drill-bit shank is axially grooved and the jaws engage in these grooves, providing excellent angular coupling while still permitting the bit to slide axially limitedly. To this end each jaw has a pointed inner end, in effect constituting a single tooth whose flanks engage the flanks of the groove. This type of arrangement does not work effectively with cylindrical- or hexagonal drill bits, as three points of purchase are not sufficient for coupling the bit to the chuck in heavy-duty use. In short order three points bite into the shank and, therefore, loosen.